The present invention relates to an exhaust gas cleaning system for an engine.
There are technologies of improving fuel economy of an engine in which an air-fuel ratio is set to a value having an air concentration higher (hereinafter, referred to as xe2x80x9ca lean air-fuel ratioxe2x80x9d) than that in the theoretical air-fuel ratio (hereinafter, referred to as xe2x80x9cthe stoichiometric air-fuel ratioxe2x80x9d), and fuel is burned under the lean air-fuel ratio.
For example, a method of injecting fuel from a position near an air-intake port in an intake pipe portion (a port injection method) which can perform lean burning under a condition of an air-fuel ratio from 20 to 25 and a method of injecting fuel directly into a cylinder (in-cylinder injection method) which can perform very lean burning under a condition of an air-fuel ratio from 40 to 50 comes into practical use. These technologies can reduce the pumping loss and the heat loss by performing lean burning, that is, by increasing an amount of intake air, and accordingly can improve fuel economy.
From the viewpoint of exhaust gas cleaning, in a case of burning under the stoichiometric air-fuel ratio condition, HC, CO and NOx in an exhaust gas can be cleaned by being oxidized and reduced at a time by a ternary catalyst. However, in the case of burning under the lean condition, it is difficult to reduce NOx because the exhaust gas is in a condition of excessive oxygen. Therefore, an exhaust gas cleaning system for an engine is proposed. In the exhaust gas cleaning system for an engine, an NOx absorbent is placed in an exhaust gas passage to absorb NOx in the exhaust gas when the air-fuel ratio of exhaust gas is lean and to release the absorbed NOx from the NOx absorbent and to reduce or contact-reduce the NOx when the air-fuel ratio is rich (a condition of excessive fuel). The air-fuel ratio temporally changed from the lean air-fuel ratio to the stoichiometric or rich air-fuel ratio with a predetermined cycle to release or reduce the NOx trapped by the NOx absorbent in order to recover the NOx trapping performance (hereinafter, generically referred to as xe2x80x9cpurgexe2x80x9d).
In such an exhaust gas cleaning system, in order to improve the fuel economy and reduce the compositions in the exhaust gas such as HC and so on, it is preferable that the period of the temporary change of the air-fuel ratio to the stoichiometric or rich air-fuel ratio condition is limited to only a period corresponding to an amount of absorbed NOx.
A technology of judging completion of NOx purge at temporary changing of the air-fuel ratio to the stoichiometric or rich air-fuel ratio is proposed in U.S. Pat. No. 2,692,380 (WO94/17291). After changing of the air-fuel ratio from the lean air-fuel ratio to the stoichiometric or rich air-fuel ratio, it is judged completion of NOx purge at the time when the air-fuel ratio detected by an air-fuel ratio sensor placed in the downstream side of the NOx absorbent changes from the lean air-fuel ratio to the rich air-fuel ratio. This is based on that until the NOx absorbed by the NOx absorbent is purged even if the air-fuel ratio in the upstream side of the NOx absorbent changes to the stoichiometric or rich air-fuel ratio, the air-fuel ratio detected by the air-fuel ratio sensor placed in the downstream side of the NOx absorbent is in a slightly lean air-fuel ratio because HC and CO in the exhaust gas flowing from the upstream side are consumed to reduce NOx, and the air-fuel ratio detected by the air-fuel ratio sensor becomes in the rich air-fuel ratio condition after completion of purging of the NOx absorbed by the NOx absorbent.
Japanese Patent Application Laid-Open. No. 10-128058 (U.S. Pat. No. 5,743,084) discloses the similar technology in which performance of an NOx trapping unit is monitored by estimating an amount of absorbed NOx from a time difference between the time when the air-fuel ratio is changed from the lean air-fuel ratio to the stoichiometric or rich air-fuel ratio to the time when the air-fuel ratio detected by an air-fuel ratio sensor placed in the downstream side of the NOx absorbent changes from the lean air-fuel ratio to the rich air-fuel ratio.
However, a waveform of the air-fuel ratio sensor placed in the downstream side of the NOx absorbent or trap (hereinafter, generically referred to as xe2x80x9cNOx trapxe2x80x9d) is affected by an oxygen storage capacity (OSC) even if an amount of NOx trapped by the NOx trap is the same. The above-mentioned technologies do not take this fact into consideration.
An object of the present invention is to provide an exhaust gas cleaning system for an engine which is capable of separately judging an amount of trapped NOx and an oxygen storage capacity of an NOx trap agent and capable of diagnosing deteroration of the NOx trap agent.
The above object can be attained by an exhaust gas cleaning system for an engine comprising an NOx trap for trapping by absorbing or adsorbing NOx in an exhaust gas when an air-fuel ratio of the exhaust gas is a lean air-fuel ratio and for releasing or reducing NOx, when the air-fuel ratio is a rich air-fuel ratio, the NOx trap being arranged in an exhaust gas passage; and an air-fuel ratio changing means for temporally changing the air-fuel ratio of the exhaust gas from a lean air-fuel ratio to the stoichiometric air-fuel ratio or a rich air-fuel ratio with a predetermined cycle, which comprises an oxygen concentration detecting means for detecting an oxygen concentration in the exhaust gas in the downstream side of the NOx trap in the exhaust gas passage; and an NOx trap amount judging means for judging an amount of trapped NOx of the NOx, trap from a detected result of said oxygen concentration detecting means in taking an oxygen storage capacity when the air-fuel ratio changing means temporally changes the air-fuel ratio of the exhaust gas to the stoichiometric air-fuel ratio or the rich air-fuel ratio.